I was editing something at work today and ran across the phrase lightening fast, with bonus e. This got my attention—I've seen this spelling plenty, but it was an unusual context. So I got to thinking about this spelling and why using lightening for lightning isn't all that unreasonable.First, it's not uncommon. Using the phrase bolt of lightening as a way to search for the term, I found 10 instances in the COCA corpus versus 206 instances of bolt of lightning. Let's call that a 4–5% hit rate. Thunder and lightning gets 156 hits; thunder and lightening gets 5, which is a somewhat lower incidence of around 3%. But it ain't zero. Point is, people do use the lightening spelling; not a lot, but it's out there in printed materials.

Second, it's not an error that spell checkers can find. Lightening is a perfectly cromulent word in its meaning of "to make or get lighter," as in lightening one's load. It's possible that a grammar checker will find the error; for example, if you write "bolt of lightening," Microsoft Word's grammar checker flags it. But in most contexts, grammar checking is not available.

Finally, it can make sense from a phonological perspective. Unless one's pronunciation is particularly precise, it's not hard to hear or make a vowel between the t and n in lightening. This is a phenomenon known as epenthesis, which is common in many dialects (mason-a-ry, ath-e-lete). And lest those of us with perfect pronunciation should feel too smug, epenthesis is the historical source of some now-standard pronunciations (famously, thunder in English got itself an epethentic d—compare Donner in German).

As with many misspellings, people don't like it. But it's an understandable one, at least. And all that said, I did fix it in the document I was editing. :-)